Mate, if they're Dabs a good firm whack just behind the eyes should quickly do it. Don't be shy - a good firm strike.
If they're plaice, the same.
If they're flounders, stick'em back cos they're watery and horrible anyway. But if you're still keen, the same as above.
As long as you've got a good 'priest' or something similar, it's not a problem really.
Best of luck.